This invention refers to a toggle lever clamping device for locking workpieces, and in particular is directed to a device for clamping metal sheet or pieces in the bodywork construction of motor vehicles; however other different uses are possible for a clamping device according to this invention.
Toggle lever clamping devices, normally used to clamp work pieces against a supporting frame, are widely known and described for example in U.S. Pat. Nos. 4,445,676; 4,921,233; 4,923,184; 5,165,670; EP 1.025.950 and EP 0.268.176.
In general, a clamping device of the aforesaid type comprises a box-shaped head to which a clamping arm is pivoted and is operatively connected, by a toggle lever mechanism, to a pneumatic, electric or other type of linear actuator to perform a tilting movement between an open position and a closed position to release, respectively to lock a workpiece.
The clamping arm is usually connected to a sliding rod of the actuator by means of a toggle lever mechanism comprising an intermediate connection link hinged to the clamping arm and to the sliding rod of the actuator.
In clamping devices of this kind, the axial movement of the actuator is usually transmitted to the clamping arm by means of a rigid connection link which, during movement must pass a dead centre condition of the toggle lever mechanism, beyond which the articulated connection between the rod of the actuator and the clamping arm assumes an apparent condition of irreversibility.
In practice, in order to overcome the dead centre position, it is necessary to generate relatively high forces when the device has to be shifted to the clamping position. In addition, to carry out the reverse movement, or opening movement, it is again necessary to exert similarly high forces, considerably higher than those required for shifting the device to the closed position, or it is necessary to exert an axial thrust by striking sharply from the outside, to make the rod of the actuator and the connection link move backwards, in order to release the rotation of the clamping lever.
The use of a rigid connection link in the toggle connection between the clamping arm and the actuator, and consequently the high forces which must be exerted to operate the clamping device, both for opening and for closing, give rise to considerable strain on the various parts of the device; this in turn causes excessive wear on the moving parts, which consequently call for periodical maintenance or replacement.
In addition to the possible breakage or irregular operation of the clamping device, the replacement of worn out parts is time consuming and involves additional costs.
In order to partly obviate these problems, U.S. Pat. No. 4,445,676 suggests the use of a toggle lever mechanism comprising an elastically yieldable connection link specially made with a long T-shaped slit to divide the same link into two hinging portions connected by an intermediate elastic arm capable of bending sideways, to allow a relative displacement between the two hinging axes of the link.
While on the one hand, the use of a simple elastically yieldable connection link partly solves the problems deriving from the strong stresses and wear on the movable parts of the device, on the other it gives rise to further problems and inconveniences in that it does not allow optimal irreversible clamping of the device which, in the presence of external forces on the clamping arm, is liable to accidentally disengage itself and release the clamped workpiece.
In addition to the impossibility of maintaining the workpiece firmly clamped, the use of a simple elastic connection link involves the risk that, after being repeatedly operated, the link can become damaged or break under strong clamping forces, rendering the device totally unserviceable.
To this purpose, during several tests it was noted that if the deflection of the elastically yielding link of the toggle lever mechanism is not precisely calculated in relation to the tightening torque to be exerted and the stresses that the same system undergoes in passing from one side to the other one of the dead centre point, or whenever a workpiece to be clamped is of a thickness even only slightly greater than contemplated, any damage of the connection link my jeopardize the irreversibility and functioning of the clamping device.
Consequently, there is a need to further improve these clamping devices by providing them with an irreversible final action in such a way as to render the clamping devices extremely reliable and much safer.
Therefore, the main scope of this invention is to provide a clamping device of the aforementioned type, which is structurally simple and provided with an elastically yielding toggle connection link allowing to operate the clamping device in a controlled condition and in a much safer and extremely reliable mode.
A further scope of this invention is to provide a clamping device as described above, provided with a toggle lever mechanism having a differentiated clamping action, capable both of controlling the axial compression of the connection link, providing at the same time an irreversible final action in the clamping condition of the device.
A further scope of the invention is to provide a clamping device of the aforementioned type provided with a toggle lever mechanism comprising an elastically yieldable connection link suitably designed to considerably increase safety and reliability of the same clamping device, by a single and inexpensive construction and such as to ensure a higher force to lock a workpiece.
According to the invention, a toggle lever clamping device has been provided, comprising:
a box-shaped head having a longitudinal axis;
a clamping arm pivotally connected at one end of the box-shaped head to rotate between open and closed conditions to lock respectively to release a workpiece;
a linear actuator at the other end of the boxshaped head; and
a toggle lever mechanism operatively connected between the actuator and the clamping arm,
said toggle lever mechanism comprising an elastically yieldable connection link having a longitudinal axis extending between hinging ends, said toggle lever mechanism having a dead centre condition beyond which the connection link maintains the clamping arm tightened against the workpiece,
wherein said elastically yieldable connection link comprises spaced apart stop means arranged to prevent an axial compression of the connection link upon reaching and overcoming said dead centre condition of the toggle lever mechanism.
According to a further aspect of the invention, the stop means for preventing the axial compression of the connection link, at and beyond the dead centre of the toggle lever mechanism, during the closure of the clamping device, comprise stop members oppositely arranged and axially extending from the hinging ends of the connection link; preferably said stop members are constructed and arranged to sequentially allow a controlled axial compression of the link upon reaching the dead centre point and a successive controlled angular deflection of same link beyond the dead centre point during the closure of the claming device.
The disposition of surface contact means at the facing front ends of the stop members, on slanting or diverging planes allows for an initial gentle and controlled elastic yielding action of the connection link during the closure of the clamping device, which enables to pass the dead centre position without any excessive force.
Still according to this invention, after passing the dead centre position, the movement of the connection link can continue in a controlled mode for a certain degree until it reaches a final stopping position in which the surface contact means of the stop members take a relative disposition, on slanted planes which stiffens the connection link, further preventing its axial compression, while allowing an angular defection of the same link; an irreversible final action and a controlled locking action of a workpiece is thereby provided, which is not obtainable with other clamping devices previously known.
The safety and reliability of the clamping device are thus considerably increased.